In traditional roofs, two common alternatives are known: the double skin roof and the inverted roof. In FR 2713687, U.S. Pat. Nos. 3,411,256 and 3,763,614 illustrations are given of double skin and inverted roofs.
Double skin roofs consist essentially of a support, an insulation layer, a watertight membrane, a secondary support and a decorative layer. Since in such a system the watertight membrane is hidden, it is difficult, in case of water leaks, to localize the infiltration and to repair it. Notwithstanding this, most double skin systems use penetrating fixings, thus increasing the risk for water leaks. Another disadvantage of the system is caused by the penetrating fixings, which form thermal bridges and increase the internal condensation risk. Internal condensation has a deleterious effect on the life of the roof. Reducing the number of fixings has consequences towards the dimensions of the fixings, possibly causing larger thermal bridges. Other disadvantages are the fact that the decorative layer always needs a secondary metal support and the fact that double skin roofs mostly are characterized by a thick build-up of layers.
The inverted roof, also known as upside down roof, was initially developed for flat roof construction. In general, insulation can be incorporated into a flat roof construction, either over or under the watertight membrane of the roof. Where the insulation system is placed on top of the watertight membrane, this is usually referred to as an inverted roof. Such a roof protects the watertight membrane from thermal cycling, effects of UV rays, weathering and physical damage. In a conventional inverted roof, the insulation is provided by foamed slabs, which are placed on top of the watertight membrane. To prevent the slabs of being blown away, or floated off, it is necessary to anchor them in place. In general, it is not possible to use mechanical fixings since such fixings normally would penetrate the watertight membrane, causing leaks. Conventionally, the insulation slabs are laid loosely on top of the watertight membrane on a flat roof; ballast with gravel or paving slabs are further added, for an additional loading of at least 50 kg/m2. This type of construction certainly cannot be described as lightweight. Also, the use of such a conventional inverted roof is restricted to low roof slopes: due to the absence of fixings, there is no resistance against sliding of the insulation and of the ballast layer.